>>715683530They got the Old Blood from the Chalice Dungeons under Byrgenwerth, when they found the Pthumerians there. It's what caused the schism between Laurence and Willem, and led the former to founding the Church. In the final game, the description for the Great Isz Chalice states that the Choir (which was created after the Church started using blood throughout Yharnam) were the first ones to interact with Ebrietas.
But yes, during most of the game's development Ebrietas' arena as well as the cave underneath Iosefka's clinic with the giants were both part of Byrgenwerth, and the Cleric Beast used to jump down from the Cathedral's second floor to fight you next to the altar (Amelia was in Hemwick).